Monitoring of radio link synchronization in a user equipment&#39;s active set

ABSTRACT

A method and arrangement in a Radio Network Controller (RNC) for monitoring radio link (RL) synchronization for each RL as soon as it is added to a User Equipment&#39;s (UE&#39;s) Active Set. When a new RL is added, each RL in the Active Set sends an Active Set Update Complete message to the RNC. When the RNC determines that a Radio Link Restore Indication message is not received from the Target NodeB serving the new RL, the RNC starts an initRlSyncT timer set for a defined time period and monitors incoming messages to determine whether the Radio Link Restore Indication message is received for the new RL before the initRlSyncT timer expires. If so, the method ends; but if not, the RNC instructs associated NodeBs and the UE to remove the new RL from the UE&#39;s Active Set.

TECHNICAL FIELD

The present disclosure relates to radio telecommunication systems and,more particularly, to a method and Radio Network Controller (RNC) formonitoring radio link (RL) synchronization in a User Equipment's (UE's)Active Set.

BACKGROUND

While performing a new Radio Link (RL) addition to a UE's Active Set(AS) following a Wideband Code Division Multiple Access (WCDMA) Event 1aor 1c, the newly added RL might not achieve complete synchronization inthe uplink (UL) or downlink (DL) or both, although an RRC Active SetUpdate Complete message may be received by the Radio Network Controller(RNC) through the other synchronized RLs in the Active Set.

In the existing methodology of RL addition, when a UE requests theaddition of a new RL by sending a Radio Resource Control RRC MeasurementReport, the RNC evaluates a handover proposal and if successful, sends aNodeB Application Protocol (NBAP) Radio Link Setup Request to the targetNodeB to configure the particular RL. Once the RNC receives an NBAPRadio Link Setup Response message, the RNC transmits the RRC Active SetUpdate message and waits for the RRC Active Set Update Complete messagefrom all the pre-existing RLs in the Active Set. However, the RNC doesnot wait for an NBAP Radio Link Restore Indication to arrive indicatingwhether the newly added RL has achieved synchronization. As a result,the newly added RL may remain in the Active Set without achievingsynchronization unless it becomes the Serving Cell and a RadioConnection Supervision (RCS) mechanism is triggered for this particularRL.

In the current implementation, it has often been observed that after anew RL has been added to a UE's Active Set either as a result of event1a or 1c, the UE sends an RRC Active Set Update Complete message withoutachieving UL synchronization for the newly added RL. This can beestablished from the fact that no NBAP Radio Link Restore Indicationmessage was received from the Target NodeB for the newly added RL. TheRRC Active Set Update message corresponding to this new RL istransmitted by all the RLs and the RRC Active Set Update Completemessage is received by the RNC from all the old RLs in the Active Setexcept the newly added one (which has not achieved synchronization).

Subsequently, an addition/replacement process for an additional new RLmay be initiated. An RRC Active Set Update (Addition) message istransmitted to add the new RL, and synchronization for the newly addedRL is also confirmed upon reception by the RNC of an NBAP Radio LinkRestore Indication message. However, the RL which was added prior tothis new RL addition procedure continues to remain unsynchronized. Thiscan be confirmed by checking the RLS_TAG corresponding to this RL in theInternal RC Supervision message. As a result, the RRC Active Set UpdateComplete message is not received by all the RLs present in the currentActive Set. Consequently, the RNC does not consider the Active SetUpdate procedure to be completed and waits for the same to arrive viaall existing RLs until an Active Set Update Timer expires.

After expiry of the Active Set Update Timer (which may range forexample, from 5-15 seconds), the UE's call is released. This occursirrespective of the fact that the serving RL had adequate signalstrength to sustain the call. Any RRC Measurement Report sent by the UEin the meantime will be buffered during this duration (wait period forRRC Active Set Update Complete message) and will not be evaluated. As aresult, any valid candidate cell that was reported foraddition/replacement could not be evaluated thereby missing theopportunity to add a stronger cell to the Active Set. Thus, theunsynchronized RL consumes physical resources such as channel element,codes, and DL power without providing any gain to the call.

This issue has severe impact on High-Speed Downlink Packet Access(HSDPA) calls as well. A new RL is added to the Active Set, but does notbecome the serving High-Speed Downlink Shared Channel (HS-DSCH) cellinitially. The RRC Active Set Update message is transmitted on thedownlink, and the RRC Active Set Update Complete message is received.However, the newly added RL could not achieve UL Synchronization, and noNBAP Radio Link Restore Indication message is received by the RNC forthis specific RL. At a later stage, when this new RL (still havingunsynchronized UL) becomes a candidate for the best cell, event 1d isreported by the UE. Accordingly, this RL becomes the serving HS-DSCHcell and the RCS supervision starts. After expiry of a HsDschRclostTtimer, the connection is released.

SUMMARY

The existing procedures thus cause several problems. The unsynchronized(UL) RL in the Active Set is leading to Active Set Update failure at alater stage since the Active Set Update Complete message is not beingreceived on the UL by all existing Active Set members. This eventuallyleads to a dropped call even though the unsynchronized RL is not thestrongest server in the Active Set. As a consequence of the abovephenomenon, during the time when the RNC is waiting to receive theActive Set Update Complete message, any RRC Measurement Report messagesfor a new cell addition or replacement are buffered and are notevaluated. As a result, strong candidates are not evaluated in time.Additionally, physical downlink resources (such as channel elements,codes, and DL power) are being wasted without achieving any gain on theuplink. For a (HSDPA) call, this leads to an overall connection releaseonce the unsynchronized RL becomes the best server and the RCS processis triggered.

The Applicant has noted that the above mentioned problems can be avoidedutilizing an arrangement in an RNC to detect the unsynchronized RL andact upon it as soon as it is added to the Active Set. The presentdisclosure introduces an arrangement in the RNC that monitors RLsynchronization for each RL as soon as it is added to the Active Setinstead of waiting until the RL becomes the best cell in the Active Setthereby triggering the RCS mechanism.

According to one embodiment, a method in an RNC is disclosed formonitoring RL synchronization in a user equipment's active set having aplurality of RLs. The method includes adding a new RL to the userequipment's active set; detecting that the new RL did not synchronizewithin a defined time period; and removing the new RL from the userequipment's active set in response to detecting that the new RL did notsynchronize within the defined time period.

In one embodiment, the detecting is performed by detecting that a RadioLink Restore Indication is not received for the new RL when pre-existingRLs have sent Active Set Update Complete messages; and in response todetecting that the Radio Link Restore Indication was not received forthe new RL, monitoring RL synchronization of the new RL even though thenew RL does not have the highest signal quality of the plurality of RLsin the active set. In a further embodiment, monitoring includes startingan initRlSyncT timer set for the defined time period; and determiningwhether the Radio Link Restore Indication is received for the new RLbefore the initRlSyncT timer expires. If so, the method ends; but ifnot, the RNC removes the new RL from the user equipment's active set.

In another embodiment, an RNC is disclosed, wherein the RNC isconfigured to monitor RL synchronization in a user equipment's activeset having a plurality of RLs. The RNC includes a non-transitory memoryfor storing computer program instructions; a processor coupled to thememory and configured to control the RNC when executing the computerprogram instructions. The processor controls a RL addition unitconfigured to add a new RL to the user equipment's active set; aconfirmation unit configured to detect that the new RL did notsynchronize within a defined time period; and a RL removal unitconfigured to remove the new RL from the user equipment's active set inresponse to the confirmation unit detecting that the new RL did notsynchronize within the defined time period.

In another embodiment, a computer program product is disclosed, whereinthe computer program product comprises software instructions stored on anon-transitory, computer-readable medium in a Radio Network Controller(RNC). When the software instructions are executed by one or moreprocessors, the RNC is caused to monitor RL synchronization in a userequipment's active set having a plurality of RLs by performing thefollowing: adding a new RL to the user equipment's active set; detectingthat the new RL did not synchronize within a defined time period; andremoving the new RL from the user equipment's active set in response todetecting that the new RL did not synchronize within the defined timeperiod.

The method and RNC of the present disclosure provide several benefits.For example, the use of valuable RAN resources such as channel elements,codes, and power is optimized by eliminating unsynchronized Radio Linksmore quickly after they are added to the Active Set. By removingunsynchronized Radio Links more quickly, adverse impacts on thesubsequent RL Addition/Replacement procedure can be avoided to a largeextent. This helps in improving retainability (reduced drops due tonon-receipt of the Active Set Update Complete message from all existingRLs in the Active Set) as well as mobility performance. For EUL/HScalls, the method and arrangement also help in optimizing RAN resourcesconsumed due to EUL non-serving cell resource allocation. HSretainability is also improved by performing the RCS procedure wheneverany new RL is added to the Active Set. This removes any unsynchronizedRL from the Active Set before the RL becomes the active server andsubsequently leads to an overall connection release due to failed ULsynchronization.

Further features and benefits of embodiments of the invention willbecome apparent from the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following section, the invention will be described with referenceto exemplary embodiments illustrated in the figures, in which:

FIG. 1 is a flow chart illustrating an exemplary embodiment of a methodin a Radio Network Controller (RNC) for monitoring radio link (RL)synchronization in a UE's active set;

FIG. 2 is a message flow diagram showing an exemplary embodiment of amethod performed following unsuccessful synchronization of a newly addedRL in a UE's Active Set;

FIGS. 3A-3B are portions of a more detailed flow chart of an exemplaryembodiment of the method of the present disclosure; and

FIG. 4 is a simplified block diagram of an RNC illustrating an exemplaryarrangement for monitoring RL synchronization in a UE's Active Set.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, like reference signs refer to like elements. Additionally, itshould be understood that the invention can be implemented in hardwareor a combination of software stored on a non-transitory memory andexecuted by a general purpose computer or microprocessor.

FIG. 1 is a flow chart illustrating an exemplary embodiment of a methodin a Radio Network Controller (RNC) 18 for monitoring radio link (RL)synchronization in a user equipment's active set having a plurality ofRLs. At step 11, a new RL is added to the UE's active set. At step 12,the RNC detects that the new RL did not synchronize within a definedtime period. At step 13, the new RL is removed from the UE's active setin response to detecting that the new RL did not synchronize within thedefined time period.

FIG. 2 is a message flow diagram showing an exemplary embodiment of aprocess performed following unsuccessful synchronization of a newlyadded RL in a UE's Active Set. Various messages are exchanged between aUE 15, a NodeB 16 inside the Active Set, a Target NodeB 17 outside theActive Set, and the RNC 18. The RNC may trigger RL Supervision as soonas a new RL is added to the Active Set.

The UE sends an RRC Measurement Report (1a) 19 to the NodeB 16, whichforwards the report to the RNC 18. The RNC sends an NBAP Radio LinkSetup Request 20 to the target NodeB 17, and the Target NodeB returns anNBAP Radio Link Setup Response message 21 corresponding to the new RL tobe added. The RNC and the Target NodeB also exchange an ALCAP EstablishRequest message 22 and an ALCAP Establish Confirm message 23. Once theRNC receives the NBAP Radio Link Setup Response message 21, the RNCsends an RRC Active Set Update (Addition) message 24 to the NodeB 16 andthe Target NodeB 17, which forward the message to the UE 15 to add theproposed RL to the Active Set. All pre-existing RLs in the Active Setreturn an RRC Active Set Update Complete message 25 to the NodeB 16,which forwards the message to the RNC.

At block 26, the RNC 18 determines that an NBAP Radio Link RestoreIndication corresponding to the newly added RL has not been receivedfrom the Target NodeB 17. Consequently, the RNC may trigger the RCSmechanism for this newly added RL, even when the newly added RL is notyet the best server. The RL may initially be tagged as “Out of Sync”,and a new user-configurable timer (initRlSyncT) for monitoring newlyadded RLs to the Active Set is initiated. If, nlnSyncInd number of goodframes (having BLER <35%) are received by the Target NodeB 17, thetarget NodeB then sends the NBAP Radio Link Restore Indication to theRNC indicating the RL has achieved UL Synchronization. If this occursbefore expiration of the initRlSyncT timer at block 27, the timer isstopped and the RL becomes synchronized. However, when the initRlSyncTtimer expires at block 27 before nlnSyncInd number of good frames arereceived by the Target NodeB (and consequently the Target NodeB does notsend the NBAP Radio Link Restore Indication to the RNC), the newly addedRL may be declared unsynchronized and subsequently removed.

Removal of the newly added RL is accomplished by sending an RRC ActiveSet Update (Removal) message 28 from the RNC 18 to the NodeB 16 and theTarget NodeB 17, which forward the message to the UE 15 indicatingremoval of the newly added RL from the Active Set. The UE returns an RRCActive Set Update Complete message 29. The RNC then sends an NBAP RadioLink Deletion Request message 30 to the Target NodeB 17 (having the ULunsynchronized RL) indicating removal of the corresponding resources.The Target NodeB returns an NBAP Radio Link Deletion Response message 31to the RNC.

It should be noted that the defined time period of the initRlSyncT timeris shorter than an Active Set Update timer that defines a time at whichthe RNC determines that adding the new RL has failed and a callassociated with the UE is released. Thus, removing the new RL when theinitRlSyncT timer expires prevents the call from being eventuallyreleased due to expiry of the Active Set Update timer.

FIGS. 3A-3B are portions of a more detailed flow chart of an exemplaryembodiment of the method of the present disclosure. At step 33, the RNC18 receives the RRC Measurement Report (1a/1c) from the UE 15. At step34, the RNC sends the NBAP Radio Link Setup Request 20 to the targetNodeB 17, and at step 35 the RNC receives the NBAP Radio Link SetupResponse message 21 from the Target NodeB. At step 36, the RNC sends theRRC Active Set Update (Addition) message 24 to the NodeB 16 and theTarget NodeB 17, which forward the message to the UE 15 to add theproposed RL to the Active Set. At step 37, the RNC receives the RRCActive Set Update Complete message 25 from all pre-existing RLs in theActive Set.

At step 38, the RNC 18 determines whether the NBAP Radio Link RestoreIndication message corresponding to the newly added RL has been receivedfrom the Target NodeB 17. If so, the newly added RL is synchronized andthe method ends at step 39. If not, the method moves to step 40 wherethe RNC may trigger the RCS mechanism for this newly added RL, even whenthe newly added RL is not yet the best server. The RL may initially betagged as “Out of Sync”, and the new user-configurable initRlSyncT timerfor monitoring newly added RLs to the Active Set is initiated. Themethod then moves to step 3B.

At step 41, the RNC continues to monitor whether it has received theNBAP Radio Link Restore Indication corresponding to the newly added RLfor the duration of the initRlSyncT timer. When the RNC receives theNBAP Radio Link Restore Indication before expiration of the initRlSyncTtimer, the method moves to step 42 where the timer is stopped and themethod ends at step 43. However, when the RNC has not received the NBAPRadio Link Restore Indication, the method moves to step 44 anddetermines whether the initRlSyncT timer has expired. If, not, themethod returns to step 41. If the timer has expired, the method moves tostep 45 where the newly added RL may be declared unsynchronized.

At step 45, the RNC 18 sends the RRC Active Set Update (Removal) message28 to the NodeB 16 and the Target NodeB 17, which forward the message tothe UE 15 indicating removal of the newly added RL from the Active Set.At step 46, the RNC receives from the UE, an RRC Active Set UpdateComplete message 29 indicating removal of the unsynchronized RL from theUE's Active Set. At step 47, the RNC sends the NBAP Radio Link DeletionRequest message 30 to the Target NodeB 17 (having the UL unsynchronizedRL) indicating removal of the corresponding resources. At step 48, theRNC receives from the Target NodeB, the NBAP Radio Link DeletionResponse message 31 indicating successful removal of the resourcescorresponding to the removed RL. The method then ends at step 43.

Once again, the defined time period of the initRlSyncT timer is shorterthan the Active Set Update timer that defines the time at which the RNCdetermines that adding the new RL has failed and the call associatedwith the UE is released. Thus, removing the new RL when the initRlSyncTtimer expires prevents the call from being eventually released due toexpiry of the Active Set Update timer.

FIG. 4 is a simplified block diagram of an RNC 18 illustrating anexemplary arrangement for monitoring RL synchronization in a UE's ActiveSet. The arrangement may be controlled by one or more processor(s) 51executing computer program instructions stored on a memory 52. Areceiving interface 53 receives various messages from the UE 15, theNodeB 16 inside the Active Set, and the Target NodeB 17 outside theActive Set, as shown in FIG. 2. Likewise, a sending interface 54 sendsthe various messages as shown in FIG. 2 to the UE, the NodeB inside theActive Set, and the Target NodeB outside the Active Set.

The processor 51 controls an RL Addition Unit 55 to add a new RL to theUE's Active Set. An NBAP RL Restore Indication Confirmation Unit 56determines whether an NBAP Radio Link Restore Indication has beenreceived for the new RL when the RNC receives an RRC Active Set UpdateComplete message from all of the pre-existing RLs in the Active Set. Ifnot, an RCS Mechanism 57 is started in order to monitor RLsynchronization of the newly added RL even when the newly added RL isnot the best server. An initRlSyncT timer 58 is also started to time themonitoring period. If the NBAP RL Restore Indication Confirmation Unit56 determines that an NBAP Radio Link Restore Indication is received forthe new RL before the initRlSyncT timer expires, the timer is stoppedand monitoring of the new RL synchronization ends. If the NBAP RLRestore Indication Confirmation Unit 56 determines that an NBAP RadioLink Restore Indication is not received for the new RL before theinitRlSyncT timer expires, the processor triggers an RL Removal Unit 59to remove the unsynchronized new RL from the UE's Active Set. Thesending interface 54 sends the RRC Active Set Update (Removal) message28, and when the RRC Active Set Update Complete message 29 is received,the sending interface sends the NBAP Radio Link Deletion Request 30 tothe Target NodeB 17 to remove corresponding resources.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

What is claimed is:
 1. A method in a Radio Network Controller (RNC) formonitoring radio link (RL) synchronization in a user equipment's activeset having a plurality of RLs, the method comprising: adding a new RL tothe user equipment's active set; detecting that the new RL did notsynchronize within a defined time period; and removing the new RL fromthe user equipment's active set in response to detecting that the new RLdid not synchronize within the defined time period.
 2. The methodaccording to claim 1, wherein detecting includes: detecting that a RadioLink Restore Indication is not received for the new RL; and in responseto detecting that the Radio Link Restore Indication was not received forthe new RL, monitoring RL synchronization of the new RL even though thenew RL does not have the highest signal quality of the plurality of RLsin the active set.
 3. The method according to claim 2, whereinmonitoring includes: starting an initRlSyncT timer set for the definedtime period; and determining whether the Radio Link Restore Indicationis received for the new RL before the initRlSyncT timer expires.
 4. Themethod according to claim 3, further comprising stopping the initRlSyncTtimer and ending the method, upon determining that the Radio LinkRestore Indication is received for the new RL before the initRlSyncTtimer expires.
 5. The method according to claim 3, wherein removingincludes sending an Active Set Update message to selected NodeBs in theradio network to remove the new RL from the user equipment's active set,upon determining that the Radio Link Restore Indication is not receivedfor the new RL before the initRlSyncT timer expires.
 6. The methodaccording to claim 5, wherein sending the Active Set Update message toselected NodeBs includes sending the Active Set Update message to NodeBscontrolling existing RLs in the active set and to a target NodeBcontrolling the new RL.
 7. The method according to claim 6, wherein thedefined time period of the initRlSyncT timer is shorter than an ActiveSet Update timer that defines a time at which the RNC determines thatadding the new RL has failed and a call associated with the userequipment is released, wherein removing the new RL when the initRlSyncTtimer expires prevents the call from being eventually released due toexpiry of the Active Set Update timer.
 8. The method according to claim7, further comprising sending a Radio Link Deletion Request to thetarget NodeB to remove resources associated with the new RL.
 9. A RadioNetwork Controller (RNC) configured to monitor radio link (RL)synchronization in a user equipment's active set having a plurality ofRLs, the RNC comprising: a non-transitory memory for storing computerprogram instructions; a processor coupled to the memory and configuredto control the RNC when executing the computer program instructions,wherein the processor controls: a RL addition unit configured to add anew RL to the user equipment's active set; a confirmation unitconfigured to detect that the new RL did not synchronize within adefined time period; and a RL removal unit configured to remove the newRL from the user equipment's active set in response to the confirmationunit detecting that the new RL did not synchronize within the definedtime period.
 10. The RNC according to claim 9, wherein: the confirmationunit is configured to detect that the new RL did not synchronize bydetecting that a Radio Link Restore Indication is not received for thenew RL; and the RNC further comprises a Radio Connection Supervision(RCS) mechanism configured to monitor RL synchronization of the new RLeven though the new RL does not have the highest signal quality of theplurality of RLs in the active set.
 11. The RNC according to claim 10,further comprising an initRlSyncT timer set for the defined time period,wherein the RCS mechanism is configured to: start the initRlSyncT timer;and determine whether the Radio Link Restore Indication is received forthe new RL before the initRlSyncT timer expires.
 12. The RNC accordingto claim 11, wherein the RNC is configured to stop the initRlSyncT timerand end the method, upon determining that the Radio Link RestoreIndication is received for the new RL before the initRlSyncT timerexpires.
 13. The RNC according to claim 11, wherein the RNC isconfigured to send an Active Set Update message to selected NodeBs inthe radio network to remove the new RL from the user equipment's activeset, upon determining that the Radio Link Restore Indication is notreceived for the new RL before the initRlSyncT timer expires.
 14. TheRNC according to claim 13, wherein the RNC is configured to send theActive Set Update message to NodeBs controlling existing RLs in theactive set and to a target NodeB controlling the new RL.
 15. The RNCaccording to claim 14, wherein the defined time period of theinitRlSyncT timer is shorter than an Active Set Update timer thatdefines a time at which the RNC determines that adding the new RL hasfailed and a call associated with the user equipment is released,wherein removing the new RL when the initRlSyncT timer expires preventsthe call from being eventually released due to expiry of the Active SetUpdate timer.
 16. The RNC according to claim 15, wherein the RNC isfurther configured to send a Radio Link Deletion Request to the targetNodeB to remove resources associated with the new RL.
 17. A computerprogram product comprising software instructions stored on anon-transitory, computer-readable medium in a Radio Network Controller(RNC), wherein when the software instructions are executed by one ormore processors, the RNC is caused to monitor radio link (RL)synchronization in a user equipment's active set having a plurality ofRLs by performing the following: adding a new RL to the user equipment'sactive set; detecting that the new RL did not synchronize within adefined time period; and removing the new RL from the user equipment'sactive set in response to detecting that the new RL did not synchronizewithin the defined time period.
 18. The computer program productaccording to claim 17, wherein a portion of the software instructionscauses the RNC to detect that a Radio Link Restore Indication is notreceived for the new RL, and in response, to monitor RL synchronizationof the new RL even though the new RL does not have the highest signalquality of the plurality of RLs in the active set.
 19. The computerprogram product according to claim 18, wherein a portion of the softwareinstructions causes the RNC to: start an initRlSyncT timer set for thedefined time period; determine that the Radio Link Restore Indication isnot received for the new RL before the initRlSyncT timer expires; and inresponse, send to NodeBs controlling existing RLs in the active set andto a target NodeB controlling the new RL, an Active Set Update messageto remove the new RL from the user equipment's active set.
 20. Thecomputer program product according to claim 19, wherein the defined timeperiod of the initRlSyncT timer is shorter than an Active Set Updatetimer that defines a time at which the RNC determines that adding thenew RL has failed and a call associated with the user equipment isreleased, wherein removing the new RL when the initRlSyncT timer expiresprevents the call from being eventually released due to expiry of theActive Set Update timer.
 21. The computer program product according toclaim 20, wherein the RNC is further configured to send a Radio LinkDeletion Request to the target NodeB to remove resources associated withthe new RL.